The invention relates to a method for operating a fuel cell system of the type having a fuel cell, a reformer for generating a hydrogen-rich gas, and a combustion chamber in which fuel cell exhaust gases are oxidized to generate heat energy required in the reformer.
A fuel cell system disclosed in U.S. Pat. No. 4,904,548, produces a hydrogen-rich gas in a reformer from liquid methanol and water, by means of steam reformation. Thereafter, the gas is supplied to the anode of the fuel cell system. The heat energy required for endothermal steam reformation is generated in a combustion chamber by oxidation of the anode exhaust and/or fuel and transferred to the reformer.
The goal of the present invention is to provide a method for operating a fuel cell system with improved overall efficiency.
This goal is achieved according to the invention by supplying an additional amount of fuel and oxygen to the reformer whenever the heat energy supplied to the reformer from the combustion chamber is insufficient, so that additional heat energy is generated directly in the reformer by the oxidation of the fuel. Oxidation of the fuel directly in the reformer in this manner has the advantage that the steam thus produced can be used for steam reformation of the fuel. As a result, less water is required. In addition, there is no need for the heat energy that would otherwise be required for evaporating the water. Finally, the dynamics of the gas generation system are improved by generating the heat energy directly in the reformer.
By controlling or regulating the additional amounts of fuel and oxygen added on the basis of reformer temperature, the overall efficiency of the fuel cell system can be further increased, since only that amount of fuel is oxidized at a given time which is required for maintaining steam reformation.
With a load change to higher power levels, the hydrogen requirement of the fuel cell increases abruptly. However, the production of hydrogen by the reformer lags behind this increased demand because of the limited dynamics involved. Therefore, for the duration of this lag, the hydrogen content in the anode exhaust is reduced, decreasing the available heat energy. However, because of the increased hydrogen demand, an increased heat requirement also develops at precisely the same time. Thus, it is advantageous during such a load increase to supply an additional amount of fuel and oxygen to the reformer for a predetermined period of time so that the reduced heat supply is compensated by the burner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.